Image scanners are available in a number of configurations, based both on physical layout and on document-handling capabilities. Typical scanner formats include flatbed and hand-held scanners as layout options, and rotary and sheetfed scanners as document-handling capabilities. Sheetfed document scanners are often used to process a large number of documents in short time, typically in connection with large-scale document printing operations or data input applications. The scanners are characterized by stationary scanning heads, together with media handling assemblies that traverse individual documents past the scanning head at relatively high speed.
To produce accurate, high-quality images, it is advantageous for an image-forming system to incorporate some form of image scanner for monitoring output image quality. The image scanner may be arranged to be off-line from the image-forming system, such as a sheetfed desktop scanner, or it may be arranged to be inline with the image-forming system. An advantage of inline scanning is that it provides opportunity to monitor and provide feedback to the image-forming system in real time for maintaining output quality. Inline scanners can be utilized in digital printing systems for internal image based control, image based diagnostics, or monitoring and verifying printed media.
One exemplary application for an inline scanner is the measurement and adjustment of the absolute printed image position upon the media, here referred to as image on paper (IOP) registration. The absolute position of a printed image relative to a media sheet is critical for an application where an extended image is formed across adjacent pages within a booklet, for example. This measurement task in turn requires accurate detection of sheet edges, providing accurate starting points for calculating exact image locations. The inline scanner media handling components must not only move the media sheets rapidly, but each sheet must be held sufficiently flat to remain within the scanner focal range, and each sheet edge must have sufficient optical contrast, enabling correct media edge detection at the image scanning location.
Since the inline scanner is composed of imperfect components, it is necessary that the associated image sensor within the scanner be calibrated on a periodic basis. Image sensor calibration, such as flat-field correction, improves quality of digital images by correcting sensor output at the pixel level, compensating for each pixel's gain and its dark current (pixel output when no input exists, or dark frame). Pixel gain measures the variation in sensor output as a function of the input. The gain is usually a linear variable and is simply the ratio of the input and output signals. Flat-field correction requires a plain, white background image (containing no cellular or fluorescent material), providing a standard input against which to measure output variations. Based on output variations, the sensor can be calibrated.
Conventional inline scanners cannot provide both a dark and light background, and therefore, cannot offer both image registration and uniformity calibration on the same device. Moreover, present sheetfed scanners include a fixed gap between two media handling elements, such as a glass plate and a baffle or media guide, for holding media within the focus of the scanner sensor (conventional scanners use full width array sensors, which have a very short depth of focus (1-2 mm)). Because of that shallow depth of focus, the printed sheet should be as close to the glass plate as possible. The fixed gap, however, causes difficulties in dealing with media of different thicknesses. Whether the gap is fixed based on a light media or heavy media thickness, the gap will be incorrect for a certain proportion of the input. A relatively narrow media gap will not accommodate thick media, leading to jamming. Conversely, a large media gap will not produce accurate images on thin media, particularly detailed images.
Therefore, a need exists for an inline scanning system that allows accurate image reading; accurate media edge detection; and periodic, automatic uniformity calibration. Moreover, an inline system that effectively handles media of different thickness, provides accurate scans of media having different thicknesses, and eliminates jamming is also needed.